Impairment or interruption of VLF communication links has occurred due to the failure of large high-power capacitors. Field experience has indicated that intense corona activity in entrapped gases within the oil dielectric precedes catastrophic dielectric breakdown and failure of the capacitor. At present, the presence of corona discharge within each capacitor is detected indirectly and with slow response times by measuring the temperature of the oil dielectric (to detect increases in temperature of the oil mass resulting from localized heating due to corona) and by monitoring the gas pressure above the oil (to determine pressure increase due to the evolution of gases resulting from corona activity).
A corona or partial discharge has been defined by the American Society for Testing and Materials as "a type of localized discharge resulting from transient gaseous ionization in an insulation system when the voltage stress exceeds a critical value. The ionization is localized over only a portion of the distance between the electrodes of the system." In the specific case of an oil-filled capacitor having polypropylene insulators and oil between parallel electrodes, partial discharge can occur in entrapped or evolved gases which displace the oil. It is important to note that the ionized gases which make up the corona are highly conductive and would result in a direct arc discharge between electrodes unless prevented from forming a complete path, as in the case with polypropylene insulating barrier.
All of the effects due to the presence of a corona discharge within the oil are not completely understood. It is known, however, that the conductive nature of the corona represents a significant perturbation in the local electric field between the electrodes somewhat analogous to the existence of a structural asperity at the metal electrode. According to the conduction and breakdown model of Thomas and Foster such an asperity results in several effects including field enhanced charge injection into the insulating fluid. In addition, free electrons in the gas can be accelerated by the electric field to velocities sufficient to cause dissociation of oil molecules forming the periphery of the gas volume. This effect, coupled with the chemical reactivity of the gas plasma, results in the evolution of additional gases, the formation of polymeric species, and the subsequent introduction into the insulating oil of particulate matter.
Heating of the surrounding materials by the corona discharge produces density gradients in the oil consequently affecting the electroviscous and electrohydrodynamic nature of the movement of the oil in an applied electric field.
All of these effects of the presence of the corona discharge within the oil dielectric, when considered in total with other effects such as the field-induced absorption of charge carriers into the polymeric insulators and the variation of oil parameters with contamination and use, present an exceedingly complex situation with multiple competitive paths leading to catastrophic breakdown of the capacitor. Nonetheless, the contributory nature of corona discharge to potential failure of a capacitor is clear and therefore the inception of corona activities should be avoided.
One known method of the detection of corona is the detection of corona-induced RF-voltages, a method which is available commercially and is in widespread use. A disadvantage of this method of VLF capacitor applications is that significant interference exists in the frequency range (typically to a few hundred kHz) normally measured by these instruments due to harmonics of the VLF fundamental and noise. Additionally, since several capacitors appear in parallel, it would be difficult to determine the location of the specific corona source resulting in RF-voltage pulses.